Such instruments are used, for example, in open spine operations to create a fixed or rigid connection between pedicle screws screwed into different vertebrae using a rod. The instrument is used to insert rods, by means of which adjacent pedicle screws are fixed relative to each other, into their respective tulip and to hold them in this position so that the rod can be secured there with a set screw. In cases where the rod is placed far above the tulip and spaced from it, e.g. due to slipped vertebrae (spondylolisthesis), large forces may be required to force the rod into the tulip or to pull a slipped vertebral body into the desired position. Instruments are known for this with which the rod is positioned toward a pedicle screw head exclusively by screw actuation of the instrument. One part of the instrument grips the head of a pedicle screw and another part of the instrument rests on a rod to be pressed into the head. The two parts are coupled to each other via a threaded mechanism and can be brought closer together by screwing action so that the rod is pressed into the tulip of the pedicle screw head or the screw is pulled towards the rod. The disadvantage of such instruments is that complex and laborious adjustment is necessary due to pure screw actuation over long feed distances. In particular, this means that the instrument is extremely difficult to disassemble for cleaning purposes, as its parts that are coupled to one another via the threaded mechanism must always be completely screwed or unscrewed.
A well-known instrument with pure screw actuation has an implant receiving area, a threaded sleeve and a front handle. Implant receiving area, threaded sleeve and handle are arranged or designed to be torque-proof and axially fixed relative to each other. The instrument also has a rod pusher and a threaded rod, which are also axially fixed but coupled so that they can rotate relative to each other. Both components are mounted in the instrument so that they can be displaced relative to the implant receiving area. The threaded rod is provided with a handle at its proximal end facing away from the rod pusher and is in engagement with the threaded sleeve. It can be screwed into and out of the threaded sleeve by turning the handle. A translation performed in this way is transferred to the rod pusher as a result of the above-described coupling to the rod pusher, while the rod pusher does not rotate. As a result, the implant receiving area and the rod pusher are axially positioned relative to each other by the threaded rod being screwed in and out of the threaded sleeve. Dismantling for cleaning purposes is time-consuming.
Since the distance over which a rod must be pressed into a pedicle screw tulip by means of the instrument is usually larger than 25 mm, e.g. 45 mm, instruments with quick adjustment have been developed. One instrument of this type is known from US 2015/0100097 A1 or US 2015/0100098 A1, for example, and comprises a latching mechanism. A threaded rod engages in a spring-loaded threaded shell or threaded section that can be positioned relatively in the radial direction. With axial pressure on the threaded rod, the threaded shell/the threaded section is shifted against the spring preload in the radial direction (is deflected), so that the threaded rod can be pushed forward in the axial direction without screw movement until a counterpressure exerted by the rod becomes too high. From this point on, a further axial positioning can be carried out by means of screwing, whereby high forces can be transmitted to the rod. As soon as the rod has reached the desired end position, a set screw is screwed into the tulip and the rod is fixed in this way. The threads are released from each other and brought out of engagement by pressing a release knob, which is operatively connected to the threaded shell or the threaded section. The threaded rod is thus released and can be withdrawn. The advantages of this mechanism are largely fatigue-free work and time savings, since the entire axial positioning does not have to be carried out by screwing. However, since these instruments are able to exert compressive forces of several 1000 N, decoupling the threaded rod from the threaded shell under pressure presents a disadvantageous risk of injuring surrounding structures or operating room staff. Although the instrument is quick to disassemble due to its latching mechanism, it has the disadvantage that unintentional complete disassembly of the instrument may occur during decoupling, which is undesirable in a surgery in which several bone screws have to be braced.
One problem that many well-known instruments have is their suitability for cleaning. As a rule, the instruments are designed as slim as possible to minimize skin incisions. Accordingly, the individual components of the instrument are placed closely together and are difficult to access, which makes them difficult to clean.